The aim of the proposed study in osmoregulation is to investigate the mechano-chemical coupling of transport events involved in that aspect of cell volume regulation best described as volume readjustment after hyposmotic stress. Volume readjustment has been observed in animals ranging from protozoa to mammals, but the mechanism is not yet understood. The process has been most extensively observed in muscle cells of the blue crab Callinectes sapidus, which will be the principle experimental model for this study. We hope to characterize the triggering event that initiates volume readjustment and determine the relationship between the rate of osmotic stress and the cell osmometric properties. We hope to investigate the relation between the primary transport events of volume readjustment and cellular metabolism, principally rates of oxygen consumption, redox state of respiratory intermediates and hydrogen ion flux. For these studies relative volume changes will be observed directly. Oxygen consumption will be monitored by Clark polarographic oxygen electrodes and the redox state of respiratory intermediates by a controlled scan spectrophotometer. This spectrophotometer will also be used to study the intracellular pH utilizing pH-sensitive dyes. Extracellular pH will be monitored and clamped with a pH stat. The studies will provide information that will lead to a better understanding of the evolutionary development of osmoregulation and the role of cell volume regulation in the physiology of health and disease.